Instrument for testing and developing the stereoptic acuity of a person&#39;s eyes



o. WITTEL ET AL 2,420,633 INSTRUMENT FOR TESTING AND DEVELOPING THE STEREOPTIC May 13,1947.

.ACUI'IY OF A PERSONS EYES Filed Sept. 2, 1943 15 Sheets-Sheet 1 OTTO WITTEL GEORGE J. KOCH JOSEPH L.BO0N

RN, vwm RH INVENTORS BY M A TTORNE S H 1] TI (ll May 13, 1947. TEL E A 2,420,633

INSTRUMENT FOR TEsT iNG AND DEVELOPING THE- STEREOPTIC ACUI'IY OF A PERSONS EYES Filed Sept. 2, 1943 15 Sheets-Sheet 2 I WITTEL I GEORGE JKOCH JOSEPH L. BOON INVENTORS o. WITTEL ETAL I 2,420,633 STING AND DEVELOPING THE STEREOPTIC ACUI'I'Y OF A PERSONS EYES Filed Sept. 2, 1943 15 Sheets-Sheet 3 May 13, 1947.

INSTRUMENT FOR TE M RN r m l5 Sheets-Sheet 4 ATTORNEYS y 1947- o. WITTEL ET AL 2,420,633

. INSTRUMENT FOR TESTING AND DEVELOPING THE STEREOPTIC ACUITY OF A PERSONS EYES Filed Sept. 2, 1943 15' Sheets-Sheet 5 FIG.6.

OTTO WITTEL GEORGE J. KOCH JOSEPH L. BOON INVENTORS W/XMM W/ ATTORNEYS May 13, 194-7. O wlTTEL ET AL 2,420,633

INSTRUMENT FOR TESTING AND DEVELOPING THE STEREQPTIC ACUI'IY OF A PERSONS EYES Flled Sept 2, 1945 15 Sheets-Sheet 6 O A ORNEYS May 3 o. WITTEL ETAL 2,420,633 INSTRUMENT FOR TESTING AND DEVELOPING THE STEREOPTIC ACUITY OF A PERSQNS EYES Filed Sept. 2, 19743 15 Sheets-Sheet 7' Y} FIG.8.

OTTO I VITTEL GEORGE J. KOCH JOSEPH L BOON INVENTORS ATTORNEYS y 13, 4 o. WITTEL ETAL INSTRUMENT FOR TESTING AND DEVELOPING THE STEREOPTIC 'ACUI'I'Y OF A PEHSONSEYES Filed Sept. 2, 1945 15 Sheets-Sheet 8 OTTO WITTEL GEORGE J. KOCH JOSEPH L BOON INVENTORS A TTORNEYS y 13, 1947- o. WITTEL ET AL 2,420,633

INSTRUMENT FOR TESTING AND DEVELOPING THE STEREOPTIC ACUI'IY OF A PERSONS EYES Flled Sept 2, 1943 15 Sheets-Sheet 9 OTTO WITTEL GEORGE J. KOCH JOSEPH L. BOON INVENTORS ORNEYS y 13, 1947- o. WITTEL ETAL INSTRUMENT FOR TESTING AND DEVELOPING THE STEREOFTIC ACUI'PY OF A PERSONS EYES Filed Sept 2, 1943 15 Sheets-Sheet l0 OTTO WITTEL GEORGE J. KOCH JOSEPH L. BOON INVENTORS May 13, 1947. wn- ET AL 2,420,633

INSTRUMENT FOR TESTING AND DEVELOPING THE STEREOPTIC ACUI'IY OF A PERSONS EYEs Filed Sept. 2, 1943 15 Sheets-Sheet 11 OTTO WITTEL GEORGE J. KOCH JOSEPH .L BOON INVENTORS BY WE /16W;

May 13, 1947. Q w L ETAL 2,420,633

INSTRUMENT FOR TESTING AND DEVELOPING THE STEREOPTIC 'ACUI'IY OF A PERSONS EYES Filed Sept. 2, 1943 15 Sheets-Sheet l2 FIG. 17.

OTTO WI TTEL GEORGE J. KOCH L. BOON INVENTORS y 13, 1947- o. WITTEL ET AL 2,420,633

INSTRUMENT FOR TESTING AND DEVELOPING THE STEREOPTIC *ACUI'I'Y OF A PERSONS EYES Filed Sept. 2, 1943 15 Sheets-Sheet l5 FIGJQ.

M717 715E714 H W E my 7 fPZ. E 4 E 389 FIG. 19A. 391 724C: 52.3720

| 352 OTTO WITTEL 727 GEORGE J. KOCH 385 JOSEPH L.BO0N

726 $23} 722 INVENTORS Q@ Q? BY W/W *P m/f%w ATTORNEYS May 13, o, wlTTEL ET AL 2,420,633

INSTRUMENT FOR TESTING AND DEVELOPING THE STEREOPTIC ACUITY OF A PERSONS EYES Filed Sept. 2, 1943 r 15 Sheets-Sheet 14 1 fi v 3 FIG. 20. 731

463 V 402 74 387 o 46Ix 384 746 395 385 OTTO WITTEL GEORGE J. KOCH JOSEPH L.BOON

\ INVENTORS o. WlTTEL ETAL ,420,633 INSTRUMENT FOR TESTING AND DEVELOPING THE STEREOPTIC May 13, 1947.

ACUITY OF A PERSONS EYES Filed Sept. 2, 1943 15 Sheets-Sheet l5 OT T O WITTEL GEORGE JKOCH JOSEPH L. BOON INVENTORS ATTORNEYS Patented May 13, 1947 INSTRUMENT FGE TESTING AND DEVELOP- ING THE STEREQPTIC ACUITY OF A PEESONS EYES Qtto Wittel, George J. Koch, and Joseph L. Boon, Rochester, N. Y., assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Application September 2, 1943, Serial No. 500,944

34 Claims.

1 The present invention relates to an instrument for testing and developing the stereoptic'acuity of a persons eyes, and particularly to an instrument designed for use in the selection and training of operators for stereoscopic and coincident range finders.

All persons do not have normal stereoscopic vision, and in fact some people suffer from a complete absence of such vision. There are instances where persons having at least normal stereoscopic vision are required, and further there are instances in which it is desired that the normal stereoscopic Vision of a person be de veloped to greater degrees in order to carry out a given job sufficiently well.

One instance particularly well suited to bring out this fact is the operation of stereoscopic and coincident range and height finders now being used by our armed forces. It is not only essential that the operators of instruments or this type possess satisfactory stereoscopic vision, but it is desirable, if not necessary, to exercise and develop the stereoscopic Vision of a normal person to a greater degree to insure maximum efiiciency. Furthermore, it is absolutely essential that the operators chosen for this job be completely trained in the handling and function of these instruments so that under fire they will be able to properly range upon the target in the fastest time, and smoothest manner, possible. Range finders as used in the field are very expensive and exceedingly complicated instruments so that it would be impractical to think of providing enough of such instruments merely for the training of raw recruits.

One object of the present invention is the provision of a stereo trainer (this term being used to designate an instrument for testing and developing a persons stereoscopic vision) for the use of testing and/or developing the stereoscopic vision of a person.

Another object is the provision of a stereo trainer for use in the selection and training of personnel for operating stereoscopic and coincident range finders.

And another object is the provision of a stereo trainer of the type set forth which is the same in operation and performance as a given range finder so that an operator can shift from the training instrument to a range finder without 2 sensing any unfamiliarity as to function or operation of the latter.

And another object is the provision of a stereo trainer of the type set forth With which the operator sees a target moving over a prescribed course in range, elevation, and azimuth, in a field con taining fixed reticle marks; and with which he can control the range and elevation of the target by means of two knobs which are similar in position and mechanical ratio to those of a given range finder.

And yet another object is the provision of a stereo trainer of the type set forth in which the range course of the target is varied automatically by shifting one of the elements of the optical system over a given path by a power-driven mechanism, and the elevation and azimuth tracking courses of the target are also varied by automatically shifting one of the optical elements, or the target, over a given path by a power-driven mechanism.

And another object is the provision of an instrument of the type set forth in which the operator can control the depth position of the target b adjustment of a range knob, and can control its elevation with respect to the reticle marks by an elevation knob, these controls being arranged to counteract the eifectiveness of the automatic mechanism tending to Vary the range and elevation of the target by manually shifting the optical element in opposition to the automatic mechanism, and/or manually altering the mechanical ratio of the automatic driving mechanism.

And still a further object is the provision of an instrument of the type set forth in which the arrangement of the automatic and manual controls for the range course and elevation tracking course is such that the operator cannot feel the movement of the optical element caused by the automatic mechanism, and which movement he is to continuously overcome in order to correct the range and elevation of the target.

And another object is to provide a trainer oi the type set forth in which the accuracy with with the observer keeps the target in the plane of the principal reticle marks is recorded automatically and continuously on a sheet of moving chart paper.

And another object is the provision of a trainer of the type set forth in which interchangeable cams control the range course of the target, and the elevation and azimuth tracking courses, whereby the variations in range, elevation, and azimuth produced by any o all of these cams can be eliminated, even while the instrument is running, by manipulation of the proper release knobs.

Another object is the provision of a trainer of the type set forth in which two distinguishable images of the target are formed and viewed as stereo separations, and the separation of the images is varied by moving a range prism along the optical axis of the system to give an apparent change in depth of the target relative to a fixed reticle which is viewed binocularly.

And a further object is to continuously move said ranging prism by a power-driven cam to continuously vary the apparent depth position of the target, and to provide a manually controlled mechanism for moving the ranging prism independently of, and in opposition to, said cam so that the operator can compensate for the movement of the ran ing prism by the cam in an effort to match the apparent depth position of the target with the reticle.

And yet another object is to provide independent power-operated and manually-operated drives for the ranging prism of the type set forth which are so arranged that the two can tend to move the ranging prism in the same or opposite directions at the same time, and the tendency of one drive will not affect the normal operation of the other; and the tendency of the poweroperated drive to move the prism will not be transmitted to the manual control knob of the manual drive in any manner to signal the operator as to how he must adjust the manual control knob to overcome the effect of the cam drive on the range prism.

The novel features that we consider characteristic of our invention are set forth with particula-rity in the a pended claims. The invention itself, however, both as to its organization and its methocs of operation, together with additional objects and advantages thereof, will best be understood from the following description of specific embodime ts when read in connection with the accompanying drawings, in which:

Fig. 1 is a top plan view, on reduced scale, of a "stereo trainer constructed in accordance with a preferred embodiment of the present invention;

Fig. 2 is an end elevation of the left end of the trainer with respect to Fig. 1;

Fig. 3 is an end elevation of the right end of the trainer with respect to Fig. 1;

Figs. 4;. 5, and 6 are elevational sectional views of successive portions of the trainer shown in Fig. 1, startim at the left and moving to the right according to the numbering, these figures when placed end to end, in order of their numbering, giving a complete sectional view of the trainer;

Fig. A is a top plan view of a portion of the instrument shown in Figs. 4 and 5;

Fig. 5B is a top plan view of a portion of the range prism carriage shown in Figs. 5 and 5A;

Fig. 7 is a transverse section of the trainer taken substantially on lines l'l of Fig, 4;

Fig. TA is a fragmentary elevation of gears which are partially obscured in Fig. '7;

Fig. 8 is a transverse section of the trainer taken substantially on lines 8-8 of Fig. 5;

Fig. 9 is a transverse section of the trainer taken substantially on line 9-9 of Fig. 6;

Fig. 10 is a transverse section of the trainer taken substantially on line l0l 0 of Fig. 6;

Fig. 11 is a diagrammatic showing of a preferred form of optical system used in the trainer shown in Fig. 1;

Fig. 12 is a detail, on enlarged scale, of a part of the driving linkage which controls the elevational tracking of the target, and showing the means for optionally disconnecting this linkage from the driving cam and providing complete manual control of the elevation position of the target;

Figs. 13, 14, and 15 are details, on enlarged scale, of a part of the azimuth tracking mechanism, and showing the manner of disconnecting the same from the power-driven control cam and assuming complete manual control of the azimuth position of the target;

Fig. 16 is a side elevational view of a stereo trainer constructed in accordance with another embodiment of the invention;

Fig. 17 is a top plan view of the trainer shown in Fig. 16 with the cover removed to show the internal mechanism;

Fig. 18 is a diagrammatic showing of the optical system used in this last-mentioned trainer;

Fig, 19 is a longitudinal section of the righthand portion of the trainer shown in Fig. 1'7 with certain parts removed to clearly show how the optical elements of the system shown in Fig. 18 are mounted in the instrument;

Fig. 19A is an enlarged sectional detail showing the clamp for holding the chart paper on the recording drum in a released position;

20 is a sectional view taken substantially on line 2Il2l of Fig. 17 and showing the pantograph linkage for the recording stylus;

Fig. 21 is a composite sectional view taken at points spaced longitudinally of the instrument, as indicated by lines 21-2! of Fig. 16, and showing the complete range course adjusting linkage, including the power-driven cam and the manual range knob control;

Fig. 22 is a sectional view taken substantially on line 22-"2 of Fig, 17, and showing the elevation and azimuth tracking mechanism for the target:

Fig. 23 is a sectional view similar to Fig. 22 with certain parts removed and with the elevational knob and its associated driving shaft shown in sections to clearly illustrate how said knob operates the differential linkage between the elevation cam and the target holder.

Like reference characters refer to corresponding parts throughout the drawings.

While the stereo trainer constituting the present invention is a research instrument primarily designed to further the study of problems related to stereoscopic range finders and range finder operation, the principles and mechanisms involved therein could just as readily be used by optometrists and oculists in their normal profession to test the stereoscopic vision of patients and to develop such a vision defective in this respect by training with such an instrument.

Essentially, the stereo trainer" constituting the present invention is a binocular film viewer which simulates a given range or height finder used by the armed forces. When looking into the instrument, the operator sees a target (a ship or an airplane) moving over a prescribed course in range, elevation, and azimuth in a field containing fixed reticle marks. He can control the range and elevation of the target by means of two knobs, similar in position and mechanical ratio to those of the range or height finder in question. As the operator turns the range knob, a recording stylus makes a continuous graph of his deviation from the true-range position of the target. The graph paper is ruled from zero to 120 seconds on the time axis, and from minus 100 U. 0. to plus 100 U. 0. E. on the range axis. U. 0. E stands for unit of error, and one U 0. E. will correspond to a given error in seconds of arc in the eyepiece field of the range or height finder in question.

Interchangeable cams control the range course of the target, and the elevation and azimuth tracking courses. Variations in range, elevation, and azimuth produced by any or all of these cams can be eliminated while the instrument is running, by manipulation of the proper release knob. Haze can be simulated by means of a silk screen placed in the region between the target film and the projection lens. A wavy sheet of transparent material moved about in this region produce the effect of atmospheric convection currents. Real targets and real atmospheric conditions can be viewed if the film target is removed and an objective-mirror system is attached. The objective focuses real targets in the plane that normally contains the photographic target.

Stereoscopic vision requires vision with two eyes. The fact that each eye sees a different image of the same scene enables us to determine the relative distances of objects in space when both eyes are used and is fundamental to the stereo trainer constituting the present inven tion.

Referring now to Figs. 1-15 inclusive, a stereo trainer constructed in accordance with a preferred embodiment of the present invention will be described. The principle of the instrument will be readily understood from an examination and explanation of the optical layout thereof shown in Fig. 11. The target slide it, diffusely illuminated by a projection lamp H and diffusing screen I2 (see Fig. 6) is imaged by an objective [3, at unit magnification, in the plane of the reticle M near the field lens l5.

For a moment let us omit consideration of the Woliaston prisms and polarizing filters in the system. The reticle marks are opaque and thus appear as black lines superimposed on the target image. The light from this target image is rendered parallel by the collimating lens it and is split into two similar beams by the beam splitter it and right-angle prism l8 which in turn direct the beam into two separate telescopes. Each telescope includes an objective 9, a reticle 2%, an eye lens 2!, and an exit pupil 22. The objectives l9 form an image of the target and reticle it on the reticles 20 of the telescopes where they are so observed in superposed relation by the eye lenses.

The observers eyes are positioned properly relative tothe exit pupil of the telescope by eyepieces E (Fig. 1) which surround and shield the eye lenses of the telescopes.

Thus far we have no stereoscopic eii'ect. Each eye sees exactly the same view and, hence, there is no depth apparent in the scene, To obtain the stereoscopic effect it is necessary to provide two separate target images and view them as stereo separations. While there are a number of ways that this can be done, in order to provide a simple and compact instrument, we ha e accomplished this by the combined use of Wollastcn prisms and polarizing filters, as will now be set forth. Normally, light vibrates in all directions. After it is polarized it vibrates in one direction only. When a polarizing filter is used for viewing an image formed by polarized light, all of the,

polarized light .will be transmitted when the filter is at the proper angle, However, if the filter is rotated through degrees, the beam will be entirely extinguished.

In the present instrument, polarization is accomplished. by Wollaston prisms which at the same time separate the beam into two component images, a left-eye image and a right-eye image, of the target, which are polarized at right angles one to the other. Referring to Fig. 11, a fixed Wollaston prism 23 is placed between the slide it! and the objective l3, and a ranging Wollaston prism 24 is positioned between the objective and the principal reticle [4 to move along the optical axis to and from the fixed prism. The beam is divided by the beam splitter l "I so that a left-eye image and a right-eye image, still oppositely polarized, are directed to each telescope.

Each telescope includes a polarizing filter 25, which are so oriented that one eye sees only one of the target images, and the other eye sees only the second.

The fixed W ollaston prism splits the beam from the target slide so that two target images are formed slightly displaced from each other and oppositely polarized. The ranging Wollaston prism (hereinafter referred to only as the range prism) tends to reconvert the two beams emergin from the fixed prism. By moving the range prism along the axis of the system, the two oppositely polarized target images can be made to approach each other, coincide, or even cross over, in the plane of the principal reticle. Then, the relative lateral movement of the two images, caused by the sliding of the range prism, results in an apparent movement in depth of the stereoscopically fused target image, If the range prism is adjusted so that the two images appear to be identically spaced with respect to the principal reticle marks, the target will appear to be at the same distance as the reticle. If the range prism is adjusted to form the left-eye image of the target to the left of the right-eye image, the target will seem to be behind the reticle. And if the left-eye image is displaced to the right, the target will appear to be in front of the reticle. When using the instrument the range prism is continuously moved back and forth relative to the fixed prism by a motor-driven cam, and the observer attempts to keep the target image at exactly the same distance as the principal reticle marks by manually adjusting the range prism. He does this by turning the range knob R. on top on the instrument, see Figs. 1 and 5.

As shown in Figs. 1, 2, and 3, the instrument as a whole comprises an elongated tubular section to either end of which is detachably connected an octagonal head (ii and 32. The instru ment in outward appearance resembles a range, or height, finder used by our armed forces, and the adjusting knobs forming a part thereof are placed and operate in the same manner as those on the finder in question, so that a person trained with this instrument is able to operate a conventional range, or height, finder with no trouble at all.

Figs. i-6 are elevational sectional views of the three successive portions of the complete instrument shown in Fig. 1, on an enlarged scale, and when placed end to end in the order of their numbering will show the complete instrument. Therefore, any optical element depicted in Fig. 11 can be located on the instrument itself with the aid of Figs. 4-6. As clearly illustrated in these figures, the lamp H, diffusing disk l2, and target slide 15, are mounted in the octagonal head 32 attached to the right end of the tube 33, looking at Fig. 1, In the head 32, a turret 33 is rotatably mounted on the stubshaft 34 to which four target slides 59, or three target slides and a field lens, are adapted to be mounted in circumferential spaced relation on the turret by means of clips 35. The periphery of the turret 33 is provided with gear teeth 35 so that the turret in effect constitutes a large spur gear. In mesh with the gear On the turret is a pinion 31 fixed to a rotatable shaft extending longitudinally of the head and operatively connected with an adjusting knob 39 extending beyond the end face of the instrument, and by means of which the turret can be manually rotated. The adjusting knob includes an indicator 4!! which cooperates with an index scale on the end face of the head 32 to indicate to the operator when any one of the targets carried by the turret is in alignment with the window 4! in the base of the head 32, see Figs, 1 and 6.

The head 3! is held onto one end of the tube 39 by four tie rods 22 in rectangular spaced relation and extending at one end through a base 53 in the head 35, and being fixed at the other end in a ring 3 telescoping with the right end of the tube. The tie rods 42, in addition to serving to hold the left-hand head 3! on the tube in assembled relation, also serve to support all of the mechanism within the tubular part of the instrument. As shown in Figs. 6 and 9, a supporting bracket 25 slidably engages the tie rods at four corners, and in addition to serving as means for holding the tie rods in spaced relation, this bracket acts as a support for the fixed Wollaston prism 23 and the objective l3, as will be set forth hereinafter.

The range p"isni is supported on a movable carriage so that it can be moved back and forth along the optical axis relative to the fixed prism, and the mounting for the range prism will now be described. As shown in Figs. 4, 5, 5A, 5B, and 8, five supporting plates lfi, 47, 48, 49, and 56 are set on the two bottom tie rods 42 in spaced relation lon itudinally of the instrument and are held against vertical movement relative thereto by setscrews threaded into a pair of spacer bars 52 extending through plates 43, ll, and 48. The top of the plates ll, and "58 are supported vertically in spaced relation by a second pair of spacer bars he plates 45 and 47 are held in proper spaced relation spacing bushings and on spacer bars 52; while the plates 41 and are held in proper spaced relation by should 5 on the spacer bars themselves. The field lens is supported in a mount fixed the former ng mounted on a shaft 59 to is d a turret carrying two or 1 icipal reticles M in circumferential spaced relation, see S, The reticle turret G3 has teeth 6! formed around its periphery and with which a pinion 6 l see Fig. 8, is adapte to mesh. -he pinion "5' is fixed to the end of a reticle shaft extending thru the left head of the intrum-e and being controlle. by a re knob on or de of the end face of ti. ead. By means of this reticle knob the reticle turret can be rotated to bring any one of the reticles thereon into alignment with the optical axis.

Between each pair of supporting plates 46 and .1 and 4S and 55 are situated a pair of horizontally spaced bushings 64 including ball races 65, see Figs. 4, 5, and 5A. Slidably mounted in these bushings on the ball races are two horizontally spaced rods 66 to the right-hand end of which is fixed a mount 6'! in which the range prism 24 is mounted, see Figs. 5, 6, and 8. The movable carriage for the range prism therefore comprises the two slidable rods 65 and the prism mount 67 carrying the prism and connecting the two rods together,

In accordance with a preferred embodiment of the present invention, through a difierential cable and pulley system, this movable carriage can be driven along the optical axis by either the range knob R or by a motor-driven range cam. When the trainer is in operation and the range knob is not turned, the carriage is moved by the range cam; the target thus appears to move in depth with respect to the reticle marks on reticle [4. By turning the range knob R, however, the operator can compensate for the motion caused by the range cam, thereby attempting to keep the range prism in the stereoscopic coincident position. The degree of accuracy with which the operator does this, is observed on a unit dial by the instructor and is recorded on a length of chart paper during a given length of run.

The differential cable and pulley system for moving the range prism by a motor-driven cam and by means of the range knob will now be described. Referring to Figs. 5, 5A, 5B, and 8, a pair of bridges l6 and H are fixed to the movable rods 66 by setscrews 12, in spaced relation longitudinally of the instrument. Fastened to, and extending between, these bridges is a square rail 73 which moves along with the rod 623. Rotatably mounted on and between the movable rail 13 and a second stationary rail l4, extendin between the two plates 41 and it, is a vertically disposed flanged pulley 15 having its periphery provided with a spiral groove '13. A cable IT is wrapped completely around the pulley and has one end fastened to one end of the rail 73 by a bolt 78 and has its other end fastened to the other end of the rail '53 by a bolt "29. The intermediate portion of the cable 11 passes around an eccentric 8G and a spring member Bl, both mounted in a cut-out in the face of the pulley, see Figs. 5 and 8, the eccentric serving as an adjustment to make the cable taut and the spring member serving to keep it in driving contact with the pulley at all times.

It will thus be appreciated that the pulle T5 is free to roll along the rails 13 and '14, and in so rolling will cause the rail 13 to move to the left when rolling clockwise, and will cause the rail to move to the right when rolling counterclockwise, looking at Fig. 5. Movement of the rail 13 causes movement or" the range prism along the optical axis by virtue of the connection between the rail 13 and the range prism carriage; and it is in this manner that the motordriven cam is adapted to cause a continuous variation in the range of the target. As shown in Fig. 4, the range cam 83, formed into a cylinder from a flat sheet of metal, is mounted on a drum 84 which is rotatably mounted on the shaft 85 extending horizontally through, and journaled in, boss 55 formed in the base 43 of the head 35. The drum 84 is connected to a large spur gear 87, also rotatably mounted on shaft 85, by a pin 83 extending horizontally from the gear through an opening in the base of the drum. This spur gear Bl is driven by the motor M through a reduction gearing comprising a pinion 89 directly geared to the drive shaft of the motor and which in turn drives a spur gear 90 fixed to a driven shaft 9| carrying a pinion 92 which engages the spur gear 81, see Figs. 7 and 7A.

The connection between the range cam 83 and the pulley I for moving the range prism over a prescribed course will now be set forth. Referring to Figs. 4, 5, 5A, 5B, and '7, a rectangular tube 93, extending longitudinally of the instrument near the bottom thereof from the head 3| towards the center of the tube 3!], is mounted adjacent to the end of the head between a pair of flanged rollers 94, which are in turn mounted on a bracket 95 fixed to the base 43 of the head, to be capable of longitudinal movement. This rectangular tube carries a roller 96 at one end which is adapted to engage the range cam The rectangular tube 83 has a clip 9? attached thereto, and a tension spring 98 connected between the clip and supporting plate 45 normally pulls the tube to the right to hold the roller 96 thereon against the range cam. Telescoping with the rectangular tube 93 is a bar 99 connected to the axle of the pulley 15 by a forked bracket I00. The rectangular tube 93 is flexibl connected to the bar 99 by a tension spring HlI connected between the left-hand end of the bar and a pin Ill2 fixed to, and extending through, the left-hand end of the rectangular tube ,93, said tension spring lying wholly within the tube, see Fig. 4. The end of the rectangular tube 93 is normally held against a stop pin I93 on the bar 99 by the tension in the spring lill so that when the tube is moving to the right the connection between the tube and bar will be positive, tending to move the axle of the pulley to the right. When the rectangular tube is moved to the left by the range cam, movement of the bar 99 in this direction will depend upon the resilient coupling provided by the spring IQI. This flexible coupling between the rectangular tube 93 and the bar 99 telescoping therewith, is necessary to account for cases in which the operator, in manipulating the range knob R tends to inadvertently move the range prism carriage beyond either of the limits of the cam driving mechanism therefor, as will be readily appreciated from the following description of the manual drive for the range prism.

To sum up the cam drive for the range prism, the range cam continually causes the axle of the pulley '55 to be reciprocated longitudinally of the instrument over a prescribed path, and this movement of the axle causes the pulle to be rotated, by virtue of its rolling contact with the stationary rail M, in opposite directions depending upon the direction of movement of the axle. As the pulley 15 rotates counter-clockwise, the cable ll wrapped therearound will cause the rail 13, and the range prism carriage connected thereto, to move to the right. And, vice versa, if the pulley 15 is rotated clockwise, by the axle thereof being moved to the right, the rail 13 and the range prism carriage will be moved to the left. As a result, the range cam is continually acting to move the range prism first in one direction and then in the other relative to the position to the stereoscopic coincidence to vary the apparent range of the target.

In using this instrument, the operator attempts to keep the range prism in the stereoscopic coincident position by compensating for the motion of the range prism caused by range cam by manipulation of the range knob R. The drive connection between the range knob R and the range prism carriage to allow for such manual com pensation of the range drive will now be described.

Referring to Figs. 5, 5A, and 513, a drive pulley I I0 having a spiral groove, and an idler pulley Ii I, are mounted in a horizontal position at opposite sides of the drive pulley l5 on spacer bars 53. On the inside face of pulley l5 there is fixedly mounted another pulley H2 having a spiral groove, this pulley being smaller than pulley l5 and disposed in a vertical plane parallel thereto. A cable H3 has one end fixed to a tie bolt [M on the pulley I Ill, makes approximately a degree wrap of the pulley, extends around the idler pulley III, then after several turns around the pulley H2 extends back to the pulley H0 where its other end is attached to a second tie bolt H5. The idler pulley MI is slidably mounted on the spacer bars 53 and is normally moved to the left, looking at Fig. 5, by a spring II6 for the purpose of keeping the cable H3 taut at all times. Thus, by turning the drive pulley [I0 in opposite directions, the pulley 15 can be rotated in either direction on the rails l3 and 14 through the pulley II2 fixed to the face thereof. Accordingly, the pulley '15 can be rotated independently of the cam drive by rotating the drive pulley Hi] to provide a manual adjustment of the range prism. It also follows that if the drive pulley Iii) is continuously rotated in the proper direction and at the proper speed when the cam drive is working, the rail l3, and the range prism connected thereto, can be held stationary. By virtue of this fact, the cam drive for the range prism can be compensated for by rotation of the pulley Ill! in the proper direction and at the proper speed, depending upon the pitch of the cam, and the range prism can be brought into, and constantly held in, the stereoscopic coincident position.

The pulley I It is connected to the manual range knob R for manual manipulation in the following manner. The range knob R is fixed to one end of a shaft E20 jcurnaled in a boss I2I carried by the control plate I22 fixed to the outside of the tube 30 by bolts I23 in covering relation with an opening I24 in the wall of the tube. Rotatably mounted on the end of the shaft I20 is an annular clutch member I25 having diametrically spaced slots I26 which are adapted to receive a pair of drive pins I21 extending above, and diametrically disposed on. the top face of the drive pulley III). This particular clutch arrangement is used to permit the range knob and its associated shaft I20 to be assembled onto, or disassembled from, the instrument by merely dropping the control plate I22 onto the tube 30, or removing it therefrom, respectively. The lower end of the shaft 226 is properly located and supported by the sliding engagement between a vertical locating pin I23, extending from the axle of the drive pulley I I8. and a bore I29 in the end of the shaft.

The actual drive connection between the range knob R. and the drive pulley I it constitutes a friction clutch in the form of a resilient friction pad I30 which is held between a collar I3 I, fixed to the shaft by a setscrew, and the top surface of the annular clutch member 25. This resilient friction pad constitutes the actual drive connection because, as pointed out above, the clutch member I25 is rotatably mounted on the shaft I23 and is connected to the shaft only through the friction clutch arrangement. As shown in Fig. 5, the resilient friction pad I33 normally tends to raise the shaft IN and the range knob 

